Conventionally, in order to raise the flexibility of base station installation in radio communication systems, particularly mobile communication systems, configurations in which the functions of a base station are distributed between two apparatuses, namely, a BBU (Base Band Unit) and an RRH (Remote Radio Head), and the BBU and RRH are physically separated, have been considered. As one mode for functional splitting schemes between a BBU and an RRH, a functional splitting scheme in which the functions of the MAC (Media Access Control) layer and higher, and the coding functions, which are a part of the physical layer functions, are performed by a BBU, and the functions of the physical layer other than the coding functions are performed by RRHs, as shown in FIG. 3, has been considered (see, e.g., Non-Patent Document 1). This functional splitting scheme is called an SPP (Split-PHY Processing) scheme.
As schemes for demodulating radio signals received in a base station or a terminal apparatus, there are soft-decision demodulation schemes in which, instead of outputting signal bits obtained by demodulation as bit values 0 or 1, the signal bits are output as real-value ratios called likelihoods, indicating the probability that a signal bit is 0 or 1 (see, e.g., Non-Patent Document 2). In a soft-decision demodulation scheme, the output obtained by demodulation is called the LLR (Log Likelihood Ratio). In general, the larger the LLR value is in the positive direction, the higher the probability that the signal bit is 1, and the lower the value is in the negative direction (i.e., the higher the absolute value), the higher the probability that the signal bit is 0.
Additionally, in a mobile communication system, the area covered by a single RRH is referred to as a cell, and in general, the coverage areas of multiple adjacent cells overlap. For this reason, when a terminal apparatus is located near a cell edge, there is a problem in that the radio signals being exchanged between the terminal apparatus and a desired RRH can encounter interference from radio signals exchanged between the terminal apparatus and the RRH of an adjacent cell, thereby significantly reducing the radio transmission rate. As a means for solving such a problem, CoMP (Coordinated Multi-Point transmission/reception) (base station cooperation) technology, in which adjacent RRHs cooperate with each other to communicate with a terminal apparatus located near the cell edges, as shown, for example, in FIG. 4, has been considered (see, e.g., Non-Patent Document 3).
In FIG. 4, there are two cooperating RRHs, but there may be two or more RRHs. The possibility of installing RRHs at a high density and having multiple RRHs constantly performing CoMP with respect to multiple terminal apparatuses, regardless of whether or not the terminal apparatuses are located at the cell edges, thereby increasing the system capacity, has been considered for use in future mobile communication systems. As one CoMP technique on an uplink (the direction from the RRHs to the BBU), a technique in which an SPP functional splitting scheme is applied, LLRs obtained in the respective RRHs are transmitted to the BBU, and the BBU combines the LLRs obtained by the respective RRHs has been considered. With this technique, the RRHs are notified beforehand, by means of the MAC layer function in the BBU, whether or not CoMP is to be performed for a terminal apparatus communicating with the RRHs (see Non-Patent Document 4).
FIG. 5 is a diagram showing an example of a system configuration of a radio communication system 1000 that performs uplink LLR-combined signal transmission in conventional SPP. The radio communication system 1000 includes a terminal apparatus 91, multiple RRHs 92-1 and 92-2, and a BBU 93. The RRHs 92-1 and the 92-2 are provided with similar structures, so the RRH 92-1 will be explained as an example.
The RRH 92-1 includes an RF (Radio Frequency) reception unit 921-1, a channel estimation unit 922-1, and a demodulation unit 923-1. The BBU 93 includes a signal separation unit 931-1, a signal separation unit 931-2, an LLR combining unit 932, a decoding unit 933, a decoding unit 934, a decoding unit 935, and a MAC layer functional unit 936.
The RF reception unit 921-1 receives signals transmitted from the terminal apparatus 91. Of the received signals, the RF reception unit 921-1 outputs reference signals to the channel estimation unit 922-1, and outputs data signals to the demodulation unit 923-1. The reference signals are signals for extracting channel information relating to the radio transmission path, and include signals that are known between the terminal apparatus and the RRHs. The data signals are signals that are to be transmitted to the BBU, including a series of signal bits.
The channel estimation unit 922-1 estimates the channel information and measures the reception quality on the radio transmission path on the basis of the reference signals output from the RF reception unit 921-1. The channel estimation unit 922-1 outputs the channel information estimation result and the reception quality measurement result to the demodulation unit 923-1. The demodulation unit 923-1 uses the channel information estimation result and the reception quality measurement result output from the channel estimation unit 922-1 to obtain LLR values (soft decision values) by performing equalization and soft-decision demodulation on the received data signals. The demodulation unit 923-1 transmits the obtained LLR values (soft decision values) to the BBU 93.
The signal separation units 931-1 and 931-2 in the BBU 93 output, to the LLR combining unit 932, the LLR values of signals transmitted from a terminal apparatus 91 for which CoMP is performed, on the basis of the information regarding whether or not CoMP is to be performed, for which notification is provided by the MAC layer functional unit 936. On the other hand, the signal separation unit 931-1 outputs, to the decoding unit 933, the LLR values of signals transmitted from a terminal apparatus 91 for which CoMP is not to be performed, and the signal separation unit 931-2 outputs, to the decoding unit 934, the LLR values of signals transmitted from the terminal apparatus 91 for which CoMP is not to be performed. In this case, the information regarding whether or not CoMP is to be performed is a flag providing notification as to whether or not CoMP is to be performed for a terminal apparatus 91 associated with each RRH 92. The value of the flag is represented by, for example, 0 or 1, such that a flag value of 0 indicates that CoMP is not to be performed for the terminal apparatus 91, and a flag value of 1 indicates that CoMP is to be performed for the terminal apparatus 91.
The LLR combining unit 932 combines the LLR values outputted from the signal separation units 931-1 and 931-2, and outputs the combined LLR values to the decoding unit 935.
The decoding unit 933 decodes the LLR values output from the signal separation unit 931-1 to restore the signal bit data (hard decision values). The decoding unit 933 outputs the restored signal bit data to the MAC layer functional unit 936.
The decoding unit 934 decodes the LLR values output from the signal separation unit 931-2 to restore the signal bit data (hard decision values). The decoding unit 934 outputs the restored signal bit data to the MAC layer functional unit 936.
The decoding unit 935 decodes the combined LLR values output from the LLR combining unit 932 to restore the signal bit data (hard decision values). The decoding unit 935 outputs the restored signal bit data to the MAC layer functional unit 936.
The MAC layer functional unit 936 outputs the signal bit data output by the decoding unit 933, the decoding unit 934, and the decoding unit 935 to a higher layer. Additionally, the MAC layer functional unit 936 determines, on the basis of the signals transmitted from a terminal apparatus 91, whether or not CoMP is to be performed on the terminal apparatus 91 that transmitted the signals. Specifically, the MAC layer functional unit 936 monitors the communications of the RRHs 92-1 and 92-2, and if the received signal intensity from a terminal apparatus 91 within the cells of the RRHs 92-1 and 92-2 is less than a threshold value, determines that CoMP is to be performed on the terminal apparatus 91. The MAC layer functional unit 936 notifies the RRHs 92-1 and 92-2, and the signal separation units 931-1 and 931-2 of the information indicating whether or not there is a terminal apparatus 91 for which CoMP is to be performed.
As described above, with an SPP base station functional splitting scheme, LLR-combined CoMP is used to input more highly reliable LLR values to the decoding unit 935 for decoding, thereby decreasing the bit errors in the radio signals and improving the radio transmission characteristics.